Recently, remarkable developments have been attained in "steel tires" in which steel cords are used for belts or carcass plies, while diversified studies have been made of adhesion between steel cord and rubber.
In general, methods for bonding a steel cord with a rubber can be roughly classified into two groups: one known as an RHS bonding system which utilizes resorcinol/hexamethylenetetramine/silica, and another one wherein metal salts, particularly, cobalt salts of organic acid are compounded with the rubber. The latter has been widely utilized and rubber compositions compounded with metal salts have been proposed as a steel cord coating rubber. For example, in Japanese Patent Application Publication No. 56-39,828, Japanese Patent Application (OPI) No. 54-52,188, Japanese Patent Application Publication No. 52-26,276, Japanese Patent Application Publication No. 54-31,511 and Japanese Patent Application (OPI) No. 58-89,631, there are disclosed rubber compositions in which cobalt naphthenate, cobalt stearate, cobalt oleate, cobalt tallate, cobalt resinate or the like is compounded as an adhesion promoter. (The term "OPI" refers to a "published unexamined Japanese patent application".)
Further, Japanese Patent Application Publication No. 56-19,820 discloses a rubber composition compounded with cobalt acetonate.
Furthermore, it has been proposed to improve the rubber properties by compounding bismaleimide. For example, in Japanese Patent Application (OPI) No. 61-238,501, there is disclosed a rubber composition having improved blow out properties under heating, wear resistant properties and gripping properties wherein 0.1 to 1.5 parts by weight of bismaleimide and 0.3 to 3 parts by weight of vulcanization accelerator are compounded per 100 parts by weight of rubber, and the weight ratio of sulfur and vulcanization accelerator is specified respectively.
In Japanese Patent Application (OPI) No. 61-14,238, there are disclosed rubber compositions providing a vulcanizate which has high strength without vulcanization reversion under high temperature vulcanization, wherein natural rubber contain bismaleimide, sulfur and at least one compound selected from dibenzothiazyldisulfide and tetraalkylthiuramdisulfide in specified amounts.
In Japanese Patent Application (OPI) No. 61-51,041, there disclosed vulcanized rubber compositions having an increased rate of vulcanization and heat resistant properties, wherein halogenized-butyl rubber contains a p-alkylphenolsulfide resin and bismaleimide and the like.
In Japanese Patent Application (OPI) No. 61-166,844, there are disclosed rubber compositions having improved scorch properties, reversion of vulcanization, heat deterioration resistance and crack resistance under flexing, wherein bismaleimide, sulfenamide, dithiophosphoric acid and sulfur are compounded with natural rubber and/or synthetic rubber.
In Japanese Patent Application (OPI) No. 62-7,739, there is disclosed improved heat resistance by compounding cobalt abietinate and linear alkylene bismaleimide.
Furthermore, British Patent No. 972,804, discloses a rubber compound having relatively good adhesion between rubber and steel wire under heat and humidity, wherein a cobalt complex of an organic acid containing boron (lately known as "Manobond C" (Trade Name) manufactured by Manchem Co.) is compounded.
Recently, demands for safety, high speed traveling properties and durability of automotive tires have been increasing so that the above conventional techniques do not always provide satisfactory adhesion.
Namely, in tires reinforced with steel cord, heat or heat under high humidity generated during running induces a deterioration of bonding force between the steel cord and rubber, causing a total break-down of the tires. According to conventional techniques, satisfactory durability of the tire is not obtainable.
Alternatively, from the standpoint of rationalization and energy saving in manufacturing, rapid vulcanization is important. Therefore, adhesion promoters have become required for promoting adhesion of rubbers to steel cords, not only for thermoresistant adhesion and thermoresistant adhesion under high humidity but also to augment initial bonding strength in a short cure time. However, adhesion promoters to meet these required characteristics have never been proposed. For example, in an RHS bonding system which utilizes resorcinol/hexamethylenetetramine/silica, both the thermoresistant adhesion and the thermoresistant adhesion under high humidity are not satisfactory. In the rubber compound containing a cobalt salt of organic acid or complex of cobalt oxyketone, the thermoresistant adhesion under high humidity is not satisfactory. Further, in the rubber compound containing a cobalt salt of organic acid and linear alkylenebismaleimide, initial bonding strength is not satisfactory and in the rubber compound containing a cobalt complex of organic acid containing boron, the thermoresistant adhesion is not satisfactory.